PCRAMs have been studied in order to accomplish an ultrahigh-density memory. PCRAMs carry out recording and erasing of data by utilizing a physical property change caused by a transition (called 1st-order phase-transition) between a crystalline state and an amorphous state of chalcogenides containing Te (for example, see Patent Literature 1 and Non Patent Literatures 1 and 2).
Usually, a recording material in PCRAM is a single-layer alloy thin film formed between electrodes by a vacuum film forming method such as sputtering or the like using a chemical composition. Because the alloy thin film is formed in such a manner, the alloy thin film has a thickness in a range of 20 nm to 50 nm, and is polycrystalline but not monocrystalline.
Since the latter half of the 1980s, chalcogenides containing Te have been studied as to its crystal structures and amorphous structures by structure analysis using X-rays or the like. The crystal structures of the chalcogenides had not been understood in details until 2004 due to the presence of Sb atoms in the chalcogenides containing Te, because X-ray diffraction or electron diffraction can hardly distinguish Te from Sb, which is next to Te and has only one less electron than Te.
This led to misunderstanding as to crystal structures of GeSbTe (225) and compounds similar to pseudo-binary composition compounds (such as 225, 147, 125 similar to GeTe—Sb2Te3), which are experimentally known to have very excellent properties and used in rewritable optical discs already on the market. It had been believed that the crystal structures of these compounds are sodium-chloride structures in which Te occupies Na sites (referred to as site a) and Ge or Sb occupies Cl sites (referred to as site b) randomly (for example, see Non Patent Literature 3).
However, the use of a synchrotron radiation X-ray analysis made it possible to perform a detailed structure analysis of the GeSbTe compound, thereby revealing that the chalcogenides including Te has a structure differed from that had been believed by then in terms of the points described below (for example, see Non Patent Literature 4).
Namely, the following points were found: (1) the crystal phase of the chalcogenides including Te is such that the Ge atoms and Sb atoms occupy the positions of Cl (site (b)) in a NaCl type simple cubic lattice not “randomly” as considered conventionally but that the positions of the Ge atoms and Sb atoms in the configuration are accurately “fixed”, and that the lattice is distorted (see FIG. 2); (2) the amorphous state of the chalcogenides including Te is not completely random and takes a twisted structure with a unit configuration in which the Ge atom inside the crystal lattice is shifted by 0.2 Å toward the Te atom from a center position (is slightly off the center thereby being ferroelectric) (see FIG. 3); and (3) stably repeatable high-speed switching is attained by utilizing restoration of this twisted unit (see FIG. 4).
Note that in FIG. 4, the structure illustrated on the left side corresponds to the structure illustrated in FIG. 2, and the structure illustrated on the right side corresponds to the structure illustrated in FIG. 3.